As is known, various types of envelopes or mailers with transparent windows exist where the window consists of a cut-out opening in the mailer substrate which is covered by a transparent patch. The transparent patch is usually secured over the cut-out opening by means of an adhesive, and may consist of any suitable film of transparent material such as glassine, cellophane, or polymeric materials including polyester, polyethylene, polycarbonate, polystyrene, and polyethylene terephthalate. The adhesive is generally applied to the mailer substrate around the perimeter of the cut-out opening to join the outer perimeter of the transparent patch thereto. The transparent patch can be secured to either the inside or outside surface of the mailer substrate.
In some modern mailing systems, a mailer is formed from a single sheet after it has been imaged by a non-impact printer. These sheets are stacked in the input hopper and fed as single plies through the printer, after which the sheets are folded to form a mailer. A window is provided to permit the name and address to show through. Added thickness caused by such window patches over die-cut window openings causes mis-shapen stacks and prevents trouble-free feeding.
The typical arrangement of such patches is disadvantageous as the transparent patch is layered on top of or below the substrate, the thickness of the window portion of the sheet is greater than that of the remainder of the sheet. As a consequence, such sheets form unstable and uneven stacks, and thus limit the maximum height to which they can be stacked. This stack-height limitation is burdensome to large scale printing operations.
Another disadvantage with mailers having a cut-out opening covered by a transparent patch is that the edges of the transparent patch often get caught by process machinery, such as sheet transport mechanisms in printers. This results in the destruction of the mailer and usually requires the machinery to be stopped so that the destroyed mailer can be removed. Moreover, when heat is employed in such process machinery, the adhesive holding the transparent patch to the mailer substrate can soften, causing the patch to become detached from the mailer substrate.
One alternative to the cut-out/transparent patch type of arrangement is to apply a transparentizing material to a predetermined portion of the cellulosic mailer substrate to thereby form a window. See, for example, U.S. Pat. No. 5,418,205 to Mehta. Such a method entails the impregnation of the cellulosic mailer substrate with transparentizing material. The spaces between the fibers of the substrate are filled by the transparentizing material. In order to make the impregnaged portion transparent, the transparentizing material must have a refractive index close to that of cellulose (1.5).
In order to produce high quality cellulosic mailers on a large, industrial scale by employing a transparentizing material, it is desirable that the transparentizing material be capable of achieving at least three important functions:
1) the ability to produce a transparentized portion which possesses a number of physical and chemical properties; PA1 2) the ability to be converted quickly from a penetrating liquid to a solid after impregnation has occurred; and PA1 3) the ability to quickly penetrate the cellulosic mailer substrate in order to fully impregnate the substrate in the shortest time possible. The drawback to producing mailers in this manner, however, is that most transparentizing materials can perform, at most, only one of the aforementioned functions.
Physically, the transparentized portion of a cellulosic mailer substrate should be physically strong and flexible (i.e., not brittle) and be receptive to inks. Chemically, the transparentizied portion should meet U.S. Postal Service specifications for reflectance (sufficient transparency to read the printing beneath the transparentized portion) and PCR ("Print Contrast Ratio"-sufficient contrast between the printing and background beneath the transparentized portion) and should have sufficient resistance to migration and/or volatilization of the transparentizing material from the place where applied on the mailer substrate such that it does not lose its transparency over time.
Conventional transparentizing materials are not capable of producing transparentized window portions which possess all of the aforementioned physical and chemical properties. U.S. Pat. No. 5,076,489 to Steidinger, for example, discloses using either wax or oil as the transparentizing material. Wax produces a brittle transparentized area which is easily marred by physical contact therewith to cause a loss of transparency. In addition, wax is not receptive to inks and therefore cannot be printed upon. Oil tends to migrate and/or volatilize easily, thus resulting in a loss of transparency over time.
In an attempt to overcome these problems, certain liquid polymerizable transparentizing compositions have been utilized. When utilizing polymerizable transparentizing compositions, the paper substrate is first rendered transparent by impregnating it with the liquid polymerizable transparentizing composition. The liquid polymerizable transparentizing composition is then cured in situ to solidify the transparentized portion. These polymerizable transparentizing compositions offer several advantages over conventional transparentizing materials, such as wax and oil, in that the end-product is usually strong and flexible and does not lose it transparency over time due to migration or/or volatilization. However, there are problems associated with the use of these polymerizable transparentizing compositions. For example, the rate at which some of the liquid polymerizable transparentizing compositions penetrate a cellulosic substrate is so slow that, after applying the transparentizing composition to the substrate, the substrate must be wound up in a tight roll for a period of time to allow the material to impregnate the substrate. See for example, U.S. Pat. No. 4,416,950 to Muller et al. Such materials are not conducive to the high-speed production of mailers having transparentized windows. In an attempt to overcome the slow rate of penetration associated with known polymerizable transparentizing compositions, solvents have been included with the polymerizable transparentizing composition to lower the viscosity thereof and thereby speed the rate of penetration of the transparentizing composition into the cellulosic mailer substrate (see, e.g., U.S. Pat. No. 4,513,056 to Vemois et al). However, the use of solvents with transparentizing materials is undesirable due to the added process machinery required to evaporate the solvent from the substrate surface and to recover the evaporated solvent. It is also known to include water or water-alcohol mixtures with the transparentizing material to increase their wetting capabilities and thus increase the rate of penetration into a cellulosic substrate. See, for example, U.S. Pat. No. 3,813,261 to Muller et al. However, the use of water with transparentizing material is typically not considered conducive to high-speed production due to the time associated with removing the water from the cellulosic substrate surface. In addition, most transparentizing agents are non-polar and are not soluble in water. Therefore, they form emulsions which are not suitable for uniform distribution of the transparentizing material onto the cellulosic substrate.
In addition, since most polymerizable transparentizing agents are non polar, they exhibit limited toner adhesion properties, making it difficult to use high speed laser printers to image the transparentized areas on the mailers. This limits the amount of transparentizing material which can be loaded onto the cellulosic substrate and therefore, limits the degree of transparency in the final transparentized cellulosic product.
Further, although many of these polymerizable transparentizing compositions are thermally cured, radiation curing is preferable for reasons of both accuracy and economy. Radiation-curable, solventless liquid paper transparentizing compositions are known, but suffer from one or more of the above-mentioned problems. For example, U.S. Pat. No. 5,418,205 discloses a solventless liquid transparentizing material which quickly penetrates the cellulosic substrate and results in a high quality transparentized portion which is strong and flexible and which does not lose transparency over time. However, the transparentizing material is non-polar. Therefore, it is insoluble in water and its wetting capabilities cannot be increased by the addition of water due to the formation of emulsions. In addition, since the transparentizing material is non-polar, it exhibits limited toner adhesion properties. Therefore, the degree of transparency in the final product is limited.
In addition, many polymerizable transparentizing compositions suffer from incomplete and/or slow crosslinking reactions. Incomplete cross-linking results in a product in which unreacted monomer or oligomer remains in the cured transparentized layer. Unreacted monomer or oligomer in the cured transparentized layer may result in tackiness. Uncured or partially cured transparentized layers which are tacky present various problems, such as blocking (i.e., when two or more forms or transparentized window areas join together and result in problematic printer feeding) and material transfer of uncured constituents to fuser rollers. In addition, unreacted monomer or oligomer in the cured transparentized layer may result in degradation of opacity of the transparentized area.
Other problems associated with polymerizable, 100% solids transparentizing compositions are odor and skin irritation on contact.
Accordingly, it is seen that a need exists in the art for a substrate suitable for use as a mailer or envelope having at least one transparentized portion which can be placed in tall, stable stacks and which does not have equipment-catching edges around the window area. Further, the transparentized portion should be capable of being produced at a rate of speed conducive to high-speed production of mailers without the need for solvents. In addition, the liquid polymerizable transparentizing compositions should exhibit good toner adhesion properties. Also, they should be amenable to curing by radiation rather than by thermal polymerization.